1. Field of the Invention
The present invention relates to an electrode catalyst containing two or more metal components, a method of preparing the same, and a fuel cell including the electrode catalyst.
2. Description of the Related Art
A fuel cell is an energy generating system in which a fuel gas is electrochemically reacted with an oxidizing gas to generate energy and the generated energy is directly converted into an electrical energy. Fuel cells can be categorized into melt carbonate electrolyte type fuel cells, phosphoric acid electrolyte type fuel cells, alkali electrolyte type fuel cells, polymer electrolyte type fuel cells, etc.
Polymer electrolyte type fuel cells include proton exchange membrane fuel cells (PEMFCs), which use hydrogen gas as a fuel, and direct methanol fuel cells (DMFCs) in which liquid methanol used as fuel is directly provided to an anode. Polymer electrolyte type fuel cells have high current density and energy conversion capability. Also, polymer electrolyte type fuel cells are operable at room temperature and can be miniaturized and hermetically fabricated, and thus are widely applicable in such fields as pollution-free automobiles, home-use power generation systems, mobile communication equipment, medical devices, military equipment, aerospace equipment and the like.
PEMFCs are an energy generating system which generates direct current as a result of an electrochemical reaction of hydrogen and oxygen. A structure of a PEMFC is illustrated in FIG. 1.
Referring to FIG. 1, a PEMFC includes an anode, a cathode, and a proton conductive membrane 11 interposed between the anode and the cathode. The proton conductive membrane 11 has a thickness ranging from 50 to 200 μm and is formed of a solid polymer electrolyte. The anode and the cathode are gas diffusion electrodes respectively including electrode substrates 14 and 15, which provide reaction gas, and catalyst layers 12 and 13 in which the reaction gas is oxidized and reduced. Hereinafter, the cathode and the anode can also be referred to as gas diffusion electrodes. In FIG. 1, a carbon sheet 16 having gas diffusion holes and functioning as a current collector is also shown.
When a hydrogen gas is provided to the PEMFC described above, an oxidation reaction occurs at the anode so that hydrogen molecules are divided into hydrogen ions and electrons. Such liberated hydrogen ions move to the cathode through the proton conductive membrane 11. By contrast, at the cathode, a reduction reaction occurs so that oxygen molecules receive electrons and are transformed into oxygen ions. The generated oxygen ions are reacted with the hydrogen ions from the anode to generate water. As illustrated in FIG. 1, in the gas diffusion electrodes of the PEMFC, the catalyst layers 12 and 13 are formed on the electrode substrates 14 and 15, respectively. The electrode substrates 14 and 15 are formed of carbon cloth or carbon paper and their surfaces are treated such that reaction gas, water which moves to the proton conductive membrane 11, and water produced resulting from the reaction of oxygen ions and hydrogen ions can easily pass through.
A DMFC has substantially the same structure as the PEMFC, except that instead of using hydrogen as the reaction gas, liquid methanol is used and provided to an anode, and an oxidation reaction is promoted by a catalyst so that hydrogen ions, electrons, and carbon dioxide are generated. Although DMFCs have lower battery efficiency than PEMFCs, DMFCs can be provided in a liquid state so that they can be more easily used with (or placed into) portable electrical applications.
Also, during a reduction reaction of oxygen molecules at the cathode of the fuel cell, Pt atoms are typically used as a catalyst for transforming oxygen molecules into oxygen ions. Although Pt atoms are a good catalyst during the oxygen reduction reaction, they are rare and expensive and thus large amount of Pt atoms cannot be commercially used. In addition, when Pt metals are used for a long period time, they agglomerate so that catalyst activity decreases. Accordingly, there is a need for a catalyst to use a smaller amount of Pt while increasing its tolerance for carbon monoxide. Also, there is a need for a catalyst that can include components other than Pt.
A cathode catalyst can be a catalyst containing two or more components (or elements) selected from Fe, Co, Ni, Cr, Cu, or the like. A carbon monoxide-tolerant catalyst can be a catalyst containing two or more components selected from Pt, Ru, Rh, Pd, Ir, W, Mo, Sn, Mn, or the like. In particular, a PtRu catalyst is primarily used in PEMFCs and DMFCs due to its excellent tolerance with respect to carbon monoxide. By using such catalysts containing two or more components, the amount of Pt used can be decreased. However, there is still an agglomeration phenomenon due to binding of Pt and/or other metals.